A DIY Idler Wheel Solution
Graham Lill has provided this short article to describe how he
made clever replacement tyres for the idler wheels in his Series 7.
This is not a simple DIY task, but for Ferrograph owners with the
necessary skills and tools, it is an interesting alternative to a set
of replacement idler wheels. Graham reports that his idlers run well,
and without excessive noise.
By Graham Lill
Around about 1990 my precious series 7 tape recorder developed the dreaded sticky idler wheel syndrome and became functionally useless. Considering the fact that I had been using it regularly since I bought it in London around 1973, I was surprised and devastated. I immediately dismantled the mechanism, extracted the gooey rubber tyred wheels, and found that one was nowhere near as bad as the other two. Nevertheless it was clear that any life remaining in it was going to be quite short. The sole use for this wheel was to provide the required dimensions for a replacement part.
Having determined that the bronze bush and metal core of the idlers were in excellent condition, I used various solvents and plastic scrapers to carefully removed all traces of rubber from them, taking particular care not to scratch the metal in any way. I determined that the outer metal surface ran perfectly true to the inner bore by rotating it on the original shaft while using a dial indicator to check this surface. I then obtained a short length of thick walled Aluminium tubing about 2 inches in diameter and machined it to a form that could be firmly and truly attached to the recovered metalware. A step was turned on the inner diameter in order that it could be assembled to run without any wobble sideways and then the outer diameter was turned to a diameter about 1mm less than the idle wheel diameter. The width of the original idler was then marked on the new blank and three "V" grooves cut at equal spacing across the face. I no longer have the original drawing and dimensions, but the "V" grooves were sized so that thin (about 2mm) neoprene "O" rings could be seated and present the required diameter on the outer surface. This is of course not critical at all, but I wanted my Ferrograph to be as close to original as possible.
The Aluminium rim was then "parted" off the end of the tube and the next one prepared in the same manner. The rims were taken to the local seals and bearings dealer where a suitable size was selected to fit firmly in the grooves without the need of adhesives.
Once I had three Aluminium rims that were a tight fit to their mating wheels, these parts were thoroughly degreased, dried and pressed together with just a smear of araldite. It is important that the parts are pressed together squarely in some sort of press otherwise a bur can be thrown up on one side of the relatively soft Aluminium and this will cause the tyre to run out of true. The "O" ring "tyres" were then put on, taking care to avoid any twists that might produce an uneven circumference. Not many Ferrograph owners will have access to a lathe but there are plenty of small machine shops capable of carrying out this work to the required accuracy for a reasonable charge.